The created TextTrack (via document.createElement) doesn't appear in the video element's textTracks property, until the track's track property accessed. var track = document.createElement('track'); track.kind = "subtitles"; track.label = "English"; track.srclang = "en"; video.appendChild(track); // track.track; // when un-commented, the line below prints 1 console.log(video.textTracks.length); // prints 0

In bug 1214027 I've found that textTracks is updated asynchronously, so if I put the console.log() inside of a setTimeout, then textTracks is defined.